While serious questions remain about the origins and source of the Yemeni "bomb plot", a clearer picture is emerging of an audacious and, as far as the CIA is concerned, a successful sting operation.

Sources familiar with the operation suggest that a CIA informant and putative suicide-bomber originally recruited by Saudi intelligence infiltrated al-Qaida in the Arabian Peninsular (AQAP) and said he wanted a bomb in order to explode an aircraft bound for the US.

The double agent was handed the latest bomb devised by AQAP and passed it on to his Saudi handlers and the CIA.

Western intelligence sources do not dispute it was a sting operation. But it seems it was more than that: the "suicide bomber"was an agent provocateur - that it is to say, there is no evidence that AQAP was already planning such a plot and that without his approach to the militant group, no such plot wouild have taken place, not yet at any rate.

"It seems the double agent was planted and offered himself up to AQAP, it was an opportunity for them to test new technology", said Tobias Feakin, director of national security and resilience at the London-based Royal United Services Institute (RUSI).

He added that claims of an AQAP-inspired plot did not seem to quite fit as both core al-Qaida and AQAP were in the process of "regrouping", adopting more of a role in support of the local population rather than planning ambitious bomb plots.

Western intelligence officials have made it clear that the "underwear" bomb, now in the hands of the FBI, will prove extremely useful in testing airport security measures, specifically over whether a bomb such as this one with no metallic content could be detected by existing screens.

The sting operation may be a morale-boosting propaganda coup. The alleged plot also served to defend the Obama administration's decision last month to step up US drones on targets in Yemen. According to the New York Times reported, the double agent also provided intelligence that led the CIA to conduct a drone strike in Yemen on Sunday that killed the AQAP leader Fahd al-Quso.

But, it seems, he did not know of the whereabouts of the bombmaker himself, Hassan al-Asiri, who must now be the prime target of a US drone attack.

Judging by the responses of sources approached about the operation, it was set up by the CIA and the Saudis, and no other intelligence agency was involved.

It does, however, raise the spectre of crying wolf - will reports of the next plot refer to a sting, or a real terrorist operation?